@jack's first tweet

Dorsey, using the handle @jack, sent out his first tweet on March 21, 2006: "just setting up my twttr"

Dorsey conceived and created the microblogging site in 2006 along with Evan Williams and Biz Stone. Dorsey served as the first CEO of Twitter, which catapulted into the limelight in 2007 at the South by Southwest festival in Austin, Texas.

The Evan Williams era

In 2008, Dorsey traded roles with fellow Twitter co-founder Williams. Williams became CEO of Twitter, taking over its day-to-day operations, and Dorsey took on the role of chairman of the board. The move was seen as a signal that Twitter was serious about turning its rapid growth into revenue, something the tech company is still struggling to do today.

Williams (left) and Stone are seen here at Twitter's old office in the South of Market neighborhood of San Francisco. In 2012, Twitter moved into a new office on Market Street.

Tweet on the Hudson

In January 2009, a US Airways flight made an emergency landing in the Hudson River. All crew and passengers survived the incident, which came to be known as the "Miracle on the Hudson." One of the first photos from the scene was shared on Twitter, breaking the news before traditional media outlets.

The photo was taken and posted by Janis Krums, who was on a ferry rushing to rescue passengers huddled on the wings of the plane. He tweeted, "There's a plane in the Hudson. I'm on the ferry going to pick up the people. Crazy." The photo spread rapidly across the Web and crashed TwitPic, the third-party app that hosted the photo.

Dick Costolo becomes CEO

In October 2010, Dick Costolo was promoted to CEO. He previously served at Twitter's chief operating officer and was instrumental in developing its fledgling business model. Williams stayed on at Twitter to focus on "product strategy," and he is currently still a member of Twitter's board. At the time Costolo took over, Twitter had 160 million registered users and roughly 90 million tweets were posted daily.

Arab Spring and 100 million milestone

Twitter played a role in what became known as the Arab Spring, a series of demonstrations and protest in 2011 across the Middle East and North Africa. Twitter was used to organize protests and rally support. It also allowed the world watch a revolution play out in real time.

Twitter also crossed several milestones in 2011. The company said in March 2011 that on average 1 billion tweets were sent every week. In September 2011, Twitter said it had 100 million monthly active users worldwide.

Obama set another record when he launched his own Twitter account, using the handle @POTUS, earlier this year. The account amassed 1 million followers in five hours, surpassing the mark set by "Iron Man" actor Robert Downey Jr., who attracted 1 million followers in just under 24 hours last year.

Twitter releases Vine

In January 2013, Twitter launched Vine. The app lets people create 6-second videos that loop indefinitely. The new service has proven its chops as a news-gathering tool, playing a role in the sharing of video during the Ferguson, Missouri, protests over police conduct in 2014.

Investors are unhappy

In June, Twitter investor Chris Sacca voiced his concerns in an 8,500-word unapologetic blog post titled, "What Twitter Can Be." Sacca said that Twitter has disappointed Wall Street more often than not and that confidence in the management has diminished since it went public in 2013.

Sacca's comments foreshadowed Costolo's departure from the top job later in June.